1. Field of the Invention
This invention is directed to novel allylic ester carbonate polymer precursors as well as to solid electrolytes derived by polymerization of such allylic compounds.
This invention is further directed to solid electrolytic cells (batteries) containing an anode, a cathode and a solid electrolyte containing a solvent and a polymer matrix which includes allylic repeating units.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features.
The solid, solvent-containing electrolyte employed in such solid batteries has heretofore contained either an inorganic matrix or an organic polymeric matrix as well as a suitable inorganic ion salt. Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are not preferred and the art typically has employed a solid electrolyte containing an organic or inorganic polymeric matrix.
Suitable organic polymeric matrices are well known in the art and are typically organic homopolymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283 or copolymers obtained by polymerization of a mixture of organic monomers. Reference is made to Fiona M. Gray, "Solid Polymer Electrolytes", VCH Publishers, Inc., New York, N.Y. 1991, the disclosure of which is incorporated herein in its entirety.
Additionally, suitable organic monomers preferably contain at least one heteroatom capable of forming donor acceptor bonds with inorganic cations (e.g., alkali ions). When polymerized, these compounds form a polymer suitable for use as an ionically conductive matrix in a solid electrolyte.
The solid electrolytes also contain an electrolyte solvent (plasticizer) which is added to the matrix primarily in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte have been art recognized to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable electrolyte solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
The solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (e.g., a mixture of a glyme and an organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the electrolyte solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (e.g., heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
While the electrolytes described above perform adequately in their intended role, there is need for improvement in the area of cycle life. Cycle life is defined as the time period during which the battery has sufficient discharge capacity. A typical cycle life provides for a slowly decreasing, but still acceptable, discharge capacity for a specified period, followed by a steep drop-off in discharge capacity to below an acceptable minimum.
Another shortcoming of solid batteries has been in the area of conductivity. Better conductivity provides improved charge transference and hence greater cumulative capacity, which is defined as the summation of the capacity of the battery over each cycle (charge and discharge) in a specific cycle life.
High capacity batteries provide for higher initial capacity but have acceptable discharge capacity over a corresponding shorter period. In addition, the drop-off in discharge capacity near the end of the cycle life is much steeper. It would therefore be a significant advance in the art to provide a battery design which can provide a longer cycle life.
Lastly, the polymer matrix used to prepare the electrolyte must be compatible with the solvent so that phase separation does not occur.